The input level on a PreSonus BlueMax is controlled by a potentiometer that will allow you to control the level you are sending to it. There are 15 pre-sets some of which are designed for peak others would be construed as RMS. Peak would act to control the maximum output allowed. RMS also controls level output but also works on the signal to increase the sustain or fatten up the sound. Hard and soft knee refer to the shape of the compression curve as viewed on a graphic display. Hard knee is shaped like a 90 degree angle. The graphic representation would like the human leg bent at the knee,i.e., hard knee. The compression effect heard is abrupt and obvious. Soft knee, on the other hand, would appear as a rounded curve when graphically displayed. And while the ultimate effect derived by soft knee as opposed to hard knee may be just as profound on the signal's level, the audible difference will be much more subtle to the ear. This is why both curves are useful in different ways and for different applications of compression to an audio signal. I hope this answer clears up the importance of Peak, RMS, and Hard Knee: Soft knee.